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LM3489 数据表(PDF) 11 Page - Texas Instruments

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部件名 LM3489
功能描述  Hysteretic PFET Buck Controller With Enable Pin
Download  25 Pages
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制造商  TI1 [Texas Instruments]
网页  http://www.ti.com
标志 TI1 - Texas Instruments

LM3489 数据表(HTML) 11 Page - Texas Instruments

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INPUT VOLTAGE - OUTPUT VOLTAGE (V)
0
20
40
60
80
100
120
140
0
5
10
15
20
25
30
35
L=10PH
L=4.7PH
L=22PH
LM3489
LM3489-Q1
www.ti.com
SNVS443B – MAY 2006 – REVISED FEBRUARY 2013
Figure 25. Propagation Delay
The operating frequency and output ripple voltage can also be significantly influenced by the speed up capacitor
(Cff). Cff is connected in parallel with the high side feedback resistor, R1. The location of this capacitor is similar
to where a phase lead capacitor would be located in a PWM control scheme. However it's effect on hysteretic
operation is much different. Cff effectively shorts out R1 at the switching frequency and applies the full output
ripple to the FB pin without dividing by the R2/R1 ratio. The end result is a reduction in output ripple and an
increase in operating frequency. When adding Cff, calculate the formula above with
α = 1. The value of Cff
depend on the desired operating frequency and the value of R2. A good starting point is 470pF ceramic at
100kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is
programmed below 2.5V, the effect of Cff will decrease significantly.
CURRENT LIMIT OPERATION
The LM3489 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the PFET or across an
additional sense resistor. When current limit is activated, the LM3489 turns off the external PFET for a period of
9µs(typical). The current limit is adjusted by an external resistor, RADJ.
The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive
input of the ISENSE comparator is the ADJ pin. An internal 5.5µA current sink creates a voltage across the
external RADJ resistor. This voltage is compared to the voltage across the PFET or sense resistor. The ADJ
voltage can be calculated as follows:
VADJ = VIN − (RADJ x 3.0µA)
(5)
Where 3.0µA is the minimum ICL-ADJ value.
The negative input of the ISENSE comparator is the ISENSE pin that should be connected to the drain of the
external PFET. The inductor current is determined by sensing the VDS. It can be calculated as follows.
VISENSE = VIN − (RDSON x IIND_PEAK) = VIN − VDS
(6)
Figure 26. Current Sensing by VDS
Copyright © 2006–2013, Texas Instruments Incorporated
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Product Folder Links: LM3489 LM3489-Q1


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